The primary objective of the proposed study is to determine the influence of molecular structure on the thermodynamics of the oxygen- hemoglobin reaction. The method of attack is to determine accurate heats of reaction for hemoglobin at various extents of reaction. The evaluation of initial and final heats of reaction for hemoglobin enable one to assess the importance of cooperative energetic effects for this reaction. Structural influences would be compared from results for monomer and tetramer molecules. Samples from different animal species would also be examined. Parallel studies with carbon monoxide and nitric oxide would be made to assess the presumed similarity with oxygen reaction. The enthalpy values determined for these reactions would enable evaluation of the entropy changes from appropriate free energy values. These quantities should have a direct relation with structural details and to proposed theoretical models for the reaction. A second objective in this work is to study the effect organic phosphates such as diphosphoglyceride have on the oxygenation reaction in hemoglobin by means of heats of reaction in the presence and absence of these materials. It may also be possible to examine the reaction between organic phosphates and hemoglobin by direct mixing experiments. The enthalpy effects for the binding of disphosphoglyceride can aid in understanding the nature of this interaction.